The ability to efficiently generate harmonics of a pump radiation is of considerable interest in various fields of science and technology. For example, it is of interest for high information density optical recording and/or playback systems, and for laser printers for facsimile and typesetting systems.
As is well known, the maximum density of information stored in an optical storage medium such as an optical disc is inversely related to the wavelength of the radiation used to record the data in the storage medium and/or the wavelength of the radiation used to read the stored data. Currently available semiconductor lasers, considered to be the most advantageous light sources for optical data storage and/or retrieval, typically emit radiation in the near infrared portion of the electromagnetic spectrum.
In order to be able to utilize solid state laser devices and yet to achieve the advantages obtainable through the use of relatively short wavelength radiation, it has been proposed to use a second harmonic generator to double the frequency (half the wavelength) of the radiation emitted by a solid state laser diode.
One approach to harmonic generation utilizes an optical waveguide formed in lithium niobate or other optically nonlinear, birefringent material such as KDP (potassium dihydrogen phosphide). See, for instance, U.S. Pat. No. 3,619,795, and Electronics, July 10, 1986, p. 36. Such harmonic generators typically require delicate independent adjustments of the ordinary and extraordinary refractive indices in the waveguide region, which may make it difficult to achieve proper phasematching over the required length of waveguide. See M. Papuchon et al., SPIE, Vol. 578, Integrated Optical Circuit Engineering II, (1985), pp. 150-155.
Another approach to harmonic generation, the one of interest herein, utilizes a multilayer planar waveguide with at least one of the layers of the structure consisting of an optically nonlinear material. The refractive indices and thicknesses of the layers are chosen such that phase-matching occurs between a predetermined mode of the pump radiation and a predetermined mode of the harmonic radiation that is generated through the interaction of the pump radiation with the nonlinear material of the structure. See, for instance, U.S. Pat. No. 3,430,061, which discloses a four-layer waveguide structure with variable voltage bias.
The coupling in the waveguide between the appropriate mode of the pump radiation and the phase-matched mode of the harmonic radiation is proportional to a coupling integral I=.intg.D(x)g.sub.f.sup.2 (x)g.sub.h (x)dx. In this expression, x is the spatial coordinate along the direction perpendicular to the layers, D(x) is the material-dependent nonlinear susceptibility, and g.sub.f (x) and g.sub.h (x) are the appropriate field amplitudes of the phase-matched modes of the pump and the harmonic radiation (e.g., TE.sub.o and TM.sub.1), respectively. In order to obtain efficient generation of harmonic radiation, it is necessary that the absolute value of the coupling integral be relatively large. This value clearly is directly related to the value of D(x). However, since for an harmonic radiation mode of odd symmetry the value of g.sub.h (x) is positive over part of the range of integration and negative over the remainder, the absolute value of I can be small, due to this sign cancellation, even if D(x) should be relatively large.
The simplest prior art multilayer waveguide harmonic generator uses a three-layer waveguide, in which an optically nonlinear core layer is bounded on both sides by an optically linear cladding layer. In such a structure, in the typical case in which a pump radiation mode of even symmetry (TE.sub.o) is phase-matched with an harmonic radiation mode of odd symmetry (e.g., TM.sub.1), the coupling integral is generally quite small, due to the above-described sign cancellation. Thus, the generation efficiency of the harmonic radiation is low in this prior art harmonic generator.
A waveguide structure that substantially circumvents the sign cancellation problem has been disclosed by H. Ito and H. Inaba, Optics Letters, Volume 2(6), June 1978, pp. 139-141. The disclosed structure consists of two core layers of relatively high refractive index that are bounded on both sides by (typically, but not necessarily, optically linear) cladding layers of relatively low refractive index. One of the two core layers consists of optically nonlinear material, whereas the other consists of optically linear material. In a structure of this type, sign cancellation can be substantially avoided since D(x) is zero in the optically linear portions of the waveguide. However, the energy of the guided harmonic radiation is typically about equally divided between the linear and the nonlinear core layers, but only the nonlinear layer contributes to the coupling. Coupling therefore is still relatively small.
Despite the fact that the value of the coupling integral for the prior art four-layer structure can be greater than for the three-layer waveguide, the conversion efficiency obtainable with the four-layer prior art device is still relatively low. For instance, in the above-referred to paper by Ito et al, it is stated that a 0.1% conversion efficiency can be obtained for a fundamental mode power of 100 mW and a 1 mm interaction length for an optimized structure of glass-ZnS-TiO.sub.2 -air. This low coupling efficiency is in part due to the inherent coupling limitations of the disclosed four-layer harmonic generator, and in part to the use of polycrystalline materials in the core layers (ZnS and TiO.sub.2). Although a polycrystalline ZnS film is known to have non-zero second order optical nonlinearity, the effective nonlinearity of polycrystalline ZnS film is substantially lower than that of single crystal ZnS.
Furthermore, in order to obtain a device having a conversion efficiency close to the calculated optimal efficiency, it is necessary that the thickness of the core layers be closely controlled, and be uniform over the length of the waveguide. The coupling efficiency in the prior art four-layer structure has a relatively strong dependence on layer thickness (see, for instance, FIG. 2 in the above referred to reference). Phase-matching over the length of the waveguide (typically several mm) may thus in practice be difficult to achieve in the prior art structure.
In view of the commerical significance of apparatus that comprises efficient harmonic generation means, harmonic generation means of the multi-layer type that can have substantially higher conversion efficiency than prior art multi-layer harmonic generators would be of considerable interest. This application discloses such a harmonic generator. The disclosed novel waveguide structure furthermore can serve as a parametric amplifier and/or oscillator.